Corrosion inhibiting pigment composition and method

ABSTRACT

A method of preparing a (Me)bis-hydrogen cyanamide composition with enhanced corrosion preventive activity, wherein Me is a divalent metal selected from the group consisting of nickel and cobalt or mixtures thereof. The method includes precipitating Ni(OH) 2  or Co(OH) 2  or mixtures thereof, in situ in a liquid reaction medium, containing a soluble cobalt or nickel salt and sodium hydroxide and a stoichiometric excess of H 2  NCN and immediately reacting the resultant precipitate with H 2  NCN. Precipitation of coprecipitated pigment compositions including the aforementioned derivatives and zinc cyanamide is also disclosed.

FIELD OF THE INVENTION

This invention relates to corrosion inhibiting pigment compositions andprocesses for making the same. More particularly, it relates to suchcompositions having particular applicability to mirror backingformulations which are based on salt derivatives of hydrogen cyanamideformed with certain transition metals such as nickel, cobalt and zinc.

BACKGROUND OF THE INVENTION

Silver layers on mirrors are extremely sensitive to the presence andcorrosive action of airborne contaminants, e.g., salt particles typicalin coastal regions, or H₂ S, NH₃ or acidic contaminants which are alwayspresent in domestic or urban environments. These contaminants, in thepresence of moisture, are able to promote oxidative processes whichoccur according to the reaction Ag-e→Ag⁺ and result in the corrosivedisintegration of the mirror's reflective layer. Specialized corrosionpreventive coating systems, known as "mirror backing" coatings areapplied in order to extend a silver mirror's service life.

It is known that non-pigmented protective coatings formulated without acorrosion inhibitor pigment component, exhibit limited anti-corrosionprotection and consequently do not provide long term service life.Therefore, corrosion inhibitor pigments are critically importantfunctional components of mirror coating systems and basically determinethe useful service life of the protected silver surface.

It is known in the prior art to use various lead salts, such as leadcyanamide as corrosion retardant pigment components of mirror backingformulations. The specific corrosion retardant activity displayed bythese compounds on Ag surfaces is attributed to the presence of leadspecies and perhaps to their S²⁻ scavenging capacity. Due to theexcellent performance of mirror backing formulations containing leadcompounds, such systems have been widely used for decades by the mirrormanufacturing industry. Efforts to develop lead-free alternatives havinganticorrosive activity for silver have been spurred by environmentalconcerns.

U.S. Pat. No. 4,707,405/1987 (Evans et al.) discloses the use of metalderivatives of hydrogen cyanamide other than lead cyanamide as corrosionretardant pigment components in mirror backing protective coatings.However that patent limits the concept exclusively to cyanamides formedby Group IIA and IIB elements such as magnesium, calcium and zinc.

There are several chemical and physical properties which a pigment gradeproduct must possess in order to function as a component of a paintformulation and/or mirror backing protective coating. Among the requiredcharacteristics are limited water solubility, moderate alkalinity andcompatibility with resins and solvents used in such formulations.Specifically the capacity to interact with, and provide inhibitiveactivity against substrate specific electro-chemical oxidant processes,which promote corrosive decomposition of Ag surfaces, are the mostimportant, and are determined by the pigment's chemical composition andstructure. In this sense the disclosures of the above identified U.S.Patent leave a need for further improved pigment systems for use inmirror backing formulations.

Zinc cyanamide is known for its valuable pigmentary proprieties and itsapplicability as anticorrosive pigment in primer formulationsrecommended i.e. for steel surface protection. In my U.S. Pat. No.5,176,894 issued Jan. 5, 1993 I disclosed a pigment grade zinc cyanamidewhich meets the quality requirements for a mirror backing protectivecoating, i.e., high assay and absence of soluble salts.

It is well known in the chemical literature that hydrogen cyanamide, adi-basic acid, forms neutral, as well as basic and acidic saltderivatives with numerous metals, inclusive of the Group IA, IIAelements and some metals of the first, second and third transitionseries, among others with Co²⁺ and Ni²⁺.

It is important to note, however, that Ni²⁺ and Co²⁺ are the onlyspecies known to form the bis-hydrogen cyanamide structures symbolizedby M (HNCN)₂ M ²⁺ =Ni²⁺, Co²⁺) where the M /NCN stoichiometrical ratiois 1:2.

Considering that the ═N--C.tbd.N moiety of any cyanamide compound, dueto its characteristic structure, is likely to generate theelectrochemically active inhibitor species (by interacting with moisturein situ in the protective coating) accountable for substrate specificcorrosion preventive activity on silver, it becomes evident that suchderivatives characterized by "bis" structure should be preferablyemployed as inhibitor pigments. In support of this observation it willbe noted that the theoretical value in weight % of the "NCN" content forZnNCN is 37.9 compared to 58.1 for Ni or Co bis-hydrogen cyanamide.

Bernard et al. [Compt. Rend. Ser. C 262(3), 282-4 (1966) report thatNi²⁺ and Co²⁺ species form cyanamide derivatives of bis-hydrogencyanamide structure and disclose a relevant preparation procedure basedon the precipitation reaction involving hexamine-nickelate or cobaltatespecies and H₂ NCN in ammoniacal medium, as follows:

    M SO.sub.4 +6NH.sub.4 OH→[M (NH.sub.3).sub.6 ].sup.2+ SO.sub.4.sup.2- +6H.sub.2 O                               1.

    [M (NH.sub.3).sub.6 ].sup.2+  SO.sub.4.sup.2- +2H.sub.2 NCN⃡M (HNCN).sub.2  H.sub.2 O↓+(NH.sub.4).sub.2 SO.sub.4 +4NH.sub.4 OH 2.

    where M =Co.sup.2+  or Ni.sup.2+

Typically the procedure is carried out by introducing H₂ NCN intoammoniacal solution of hexamine-nickelate or -cobaltate and bysubsequent agitation of the system for 18 hours at pH=7.5. A similarprocedure is disclosed, specified exclusively for Ni²⁺ cyanamide inExample #4 of Japanese Patent Nr. SHO 29-8020/12.07.54. The process isperformed in one hour by simultaneously introducing nickel sulfatesolution and ammonia gas into hydrogen cyanamide solution, at 25°-30° C.and by keeping the pH of the system at 7.5 to 8.5.

The NiSO₄ /H₂ NCN=1:1 molar ratio, recommended, quite surprisingly, bythis Japanese patent, fails to consider the bis-hydrogen cyanamidestructure of the intended product and represents a basicstoichiometrical error (See reaction 2.), which results in particularlylow yield (of about 53% in Ni(HNCN)₂ H₂ O) based on the disclosed value,and the correspondingly high amount (practically 50%) of unconvertedNiSO₄ dissolved and lost in the process water.

As expected, the disclosed value of the obtained product's nitrogencontent (33.2%N) is consistent with bis-hydrogen cyanamide composition,however of a relatively poor quality, which is a direct consequence, aswell, of the employed inadequate NiSO₄ /H₂ NCN molar ratio. For the samereason under the final conditions of the process (absence of H₂ NCN,high Ni²⁺ concentration, Ph˜8.0) basic divalent nickel salts alsoprecipitate which, by subsequent dehydration are converted into darkcolored, inactive inclusions in the final products.

In addition to the above-exemplified shortcomings, there are inherentlimitations of the manufacturing procedures known by the prior art, allspecifically related to the precipitation reaction involvinghexamine-nickelate or -cobaltate and H₂ NCN, respectively to theemployment of ammonia as pH control reagent. Beside the inconveniencecaused by the volatility of NH₃ at the recommended pH value, whichrequire the employment of protective technologies, the followinglimitations are observed:

1. Reaction 2, a typical process of precipitation involving dissolvedhexamine nickelate or cobaltate species, reaches an equilibrium whichobstructs the completion of the direct reaction (basically the formationof the product by precipitation) to the desirable extent, even atsubstantial stoichiometrical excesses of H₂ NCN. This undesirablecharacteristic of the reaction system is a direct consequence of thepresence of ammonium salts, soluble by-products formed according toreaction 2, capable to prevent the complete precipitation of Ni²⁺ orCo²⁺ species as cyanamides. Thus the yield of the process issubstantially reduced and the resulting process water (mother liquor andwash water) contains large amounts of irrecuperable Ni²⁺ species as wellas undesirable ammonium salts.

2. As the complete removal of the soluble by-products (usuallyaccomplished by extensive washing) is the critical phase of anycorrosion retardant pigment manufacturing process which essentiallydetermines the quality of the final product, processes that use ammoniafor pH control, for aforementioned reasons, result in large amounts ofnon-recyclable, environmentally hazardous process water with high Ni²⁺or Co²⁺ contents.

SUMMARY OF THE INVENTION

Consistent with the above disclosed considerations, it is a principalobject of the present invention to provide inhibitor pigment gradeproducts and coprecipitated pigment compositions characterized bybis-hydrogen cyanamide structures, or containing such products, andprocedures to manufacture the same. The pigment compositions producedaccording to the subsequently disclosed procedures provide corrosioninhibitive activity, specifically on silver substrates, and are suitableto be employed in "mirror backing" paint and coating formulations.

Coprecipitated pigment compositions comprising Ni²⁺ or Co²⁺ bis-hydrogencyanamides and various transition metal cyanamides; i.e., Pb²⁺, Cd²⁺,Cu²⁺, Ag⁺, etc., can be obtained according to the subsequently disclosedprocedures. However, without any intent to limit the applicability ofthe present invention, the relevant Examples are limited to Co²⁺, Ni²⁺,and Zn²⁺ coprecipitated cyanamides.

As subsequently disclosed, Ni²⁺ and Co²⁺ bis-hydrogen cyanamide, orcompositions comprising such derivatives coprecipitated with transitionmetal cyanamides, more specifically zinc cyanamide, are produced atpigment grade quality (high assay, soluble salt contaminant-freeconditions) according to an environmentally compatible procedure whichresults in heavy metal-free waste waters.

Briefly summarized, the invention contemplates a method of preparingpigment grade Ni²⁺, Co²⁺ (or Ni²⁺ +Co²⁺)bis-hydrogen cyanamidederivatives with enhanced corrosion inhibitive activity on silver. Themethod includes reacting, "in situ" freshly precipitated Ni(OH)₂,Co(OH)₂ or mixtures thereof, with stoichiometrical excess of H₂ NCN.

Coprecipitated pigment compositions containing bis-Hydrogen cyanamidederivatives and zinc cyanamide are also produced by introducingdispersed and hydrated ZnO into the reaction system and reacting it withH₂ NCN at appropriate stoichiometrical ratios.

BRIEF DESCRIPTION OF DRAWINGS

IR Spectra characterizing Ni²⁺ or Co²⁺ or (Co²⁺ + Ni²⁺) bis-hydrogencyanamide compounds and coprecipitated pigment compositions based onZnNCN and the aforementioned bis-hydrogen cyanamide derivatives in 10/1molar ratio, are presented in FIG. 1 and FIG. 2, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The above presented limitations, which are characteristic to themanufacturing procedures known by the prior art, are eliminatedaccording to the present invention by producing high quality pigmentgrade Ni²⁺ or Co²⁺ bis-hydrogen cyanamide based on the heterogeneousreaction involving insoluble and freshly precipitated Ni(OH)₂ or Co(OH)₂and H₂ NCN, as follows:

    M SO.sub.4 +2NaOH→M (OH).sub.2 ↓+Na2SO.sub.4 3.

    M (OH).sub.2 ↓+2H.sub.2 NCN→M (HNCN).sub.2  H.sub.2 O↓4.

    where M =Ni.sup.2+  or Co.sup.2+

Such pigment grade quality Ni²⁺ or Co²⁺ bis-hydrogen cyanamidestypically are characterized by high N (or cyanamide) content, virtualabsence of insoluble contaminants and light green or light brown color,respectively.

Both, Ni²⁺ and Co²⁺ species are characterized by similar chemicalbehavior under the conditions of reaction 3 and 4; both Co(OH)₂ andNi(OH)₂ are insoluble in excess of NaOH or in the presence of solubleby-products of reaction 3, and if freshly precipitated, reactinstantaneously and quantitatively with H₂ NCN.

As above indicated, in contrast to reaction 2, reaction 4 is adiffusionally controlled heterogeneous process which takes place at theM (OH)₂ liquid interface and converts the freshly precipitated solidprecursor into the solid final product. It is observed that the reactionplausibly occurs by nucleophile substitution, where the .sup.(-) HNCNanion is actually the reactive species, formed by dissociation favoredin alkaline medium, as follows:

    H.sub.2 NCN+OH.sup.- ⃡.sup.(-) HNCN+H.sub.2 O  5.

As a consequence of the above presented considerations, the rate ofreaction 4 is determined by the magnitude of the:specific surface areaof the hydroxide phase, the .sup.(-) HNCN concentration and, mostimportantly, the alkalinity of the medium. It will be observed, however,that the effect of the medium's alkalinity on the reaction rate is acomplex one; it does favorably regulate the concentration of the.sup.(-) HNCN anion but also promotes the well known dimerization of H₂NCN, in this case an undesirable process, competitive with reaction 4.

Under the conditions determined by reaction 4, processes could occur, aswell, which are undesirable in reference to the object of the presentinvention and adversely affect the final products' quality.

In this respect, it will be observed that under mild basic conditions,Co²⁺ species specifically tend to turn into insoluble basic salts ofCo(OH)x type (where x=NO₃ --, SO₄ 2--), which although are readilyconvertible into Co(OH)₂ with appropriate excess of NaOH, do notthemselves react according to reaction 4.

Equally undesirable is the tendency of freshly precipitated Co(OH)₂ toundergo slow oxidation on exposure to air, yielding dark colored andnon-reactive Co(OH)₃, according to the following reaction:

    4Co(OH).sub.2 +O.sub.2 +2H.sub.2 O→4Co(OH).sub.3 ↓6.

Such coprecipitated non-reactive species are converted by dehydrationinto darkly colored oxides, which, as inclusion contaminants, degradethe typically light color and reduce the cyanamide content of the finalproducts.

The incomplete conversion according to reaction 4 of the Co(OH)₂ orNi(OH)₂ reactive precursor species, due to less than optimal ⁻ HNCNconcentration in the reaction medium, as well, adversely affects thefinal product's quality.

The occurrence of these undesirable side reactions are preventedaccording to the present invention by selecting appropriate processparameters, as subsequently disclosed.

In this respect, it is observed, that reactions 3 and 4 according to thepresent invention are preferable carried out by instantaneously reactingNi(OH)₂ and/or Co(OH)₂, freshly precipitated in situ in the reactionmedium, with an appropriate excess of H₂ NCN.

During precipitation, which is performed in 15 to 30 minutes, thealkalinity and temperature of the reaction medium is kept at pH=8 to9.0, preferably 8.5, and 15°-25° C., respectively.

The above specified optimal reaction conditions are maintained bysimultaneous and individual introduction of the raw materials, atappropriate rates, into the reaction medium. The method of thisinvention also includes the further steps of filtering the reactionproduct, washing the filtered product with water to a substantiallysalt-free condition and subsequently drying the product.

With respect to the relevant IR spectrums presented in FIGS. 1 and 2, itwill be noted that the presence of a narrow, intense band at 3300 cm⁻¹,as well as the absence of any absorption band at the 775⁻¹ region, arethe specific features characteristic to the bis-hydrogen cyanamidestructure. The triplet band situated roughly at the 2000-2200 cm⁻¹region is typical for cyanamides.

COMPARATIVE EXAMPLE 1

In order to exemplify the aforementioned inherent limitations of themanufacturing procedures known by the prior art, Ni(HNCN)₂ H₂ O wasproduced following basically the recommendations disclosed in Example #4of the above-identified Japanese Patent, with some exceptions, asfollows:

adequate Ni²⁺ /H₂ NCN=1:2,2 molar ratio was applied, correspondent tothe bis-hydrogen cyanamide structure of the final product

concentrated aqueous NH₃ solution was employed as pH control reagent

all three reagent solutions involved were simultaneously introduced intothe reactor at appropriate rate and the total volume of the aqueousphase was comparatively reduced to practical values

The process was carried out in 30 minutes by simultaneously introducingSolutions (A), (B) and (C) containing the appropriate amounts of rawmaterials [(A)--394.1 g. or 1.5 moles of NiSO₄ 6H₂ O in 900 ml.;(B)--138.6 g. or 3.3 moles of H₂ NCN in 500 ml.; (C)--57.0 g. or 3.3moles of NH₃ in 500 ml.]into intensively stirred H₂ O at 25°-30° C.

A temporary stoichiometrical excess of H₂ NCN was initially generatedand kept during the precipitation process, by introducing "ab initio"10% of Solution (B) into the reactor. Subsequently, all during theprecipitation process, Solutions (A) and (B) were introduced atidentical rates; however, the delivery rate of (C), the ammoniasolution, was adjusted as required to keep the reaction medium's pH=7.5to 8.5 as recommended by the aforementioned Japanese patent.

The precipitation process was completed by fixing the reaction system'spH=8.5 and stirring for an additional 30 minutes. Subsequently, thevivid green precipitate obtained was filtered, washed to salt-freeconditions and dried at 70° C. for 12 hours. The experimental resultsare presented in Table #1.

                  TABLE 1                                                         ______________________________________                                        Measured Parameters Found Values                                              ______________________________________                                        Product's appearance                                                                              vivid green powder                                        Yield               191.5 g.; 80.5%                                           Ni as Ni %          40.0                                                      N as N %            33.3                                                      Ni (HNCN).sub.2.H.sub.2 O %                                                                       83.7 calculated, based                                                        on N %                                                    H.sub.2 O %         9.8 calculated                                            Non-soluble impurities,                                                                           6.5 calculated                                            possible NiO %                                                                N/Ni ratio          3.49                                                      Recovered Process Water:                                                      Volume              2320 ml.                                                  Ni.sup.2+  Concentration                                                                          5.78 g./l.                                                Total Ni.sup.2+  Content                                                                          13.4 g.                                                   Recovered wash water:                                                         Volume              6500 ml.                                                  Total Ni.sup.2+  Content                                                                          2.67 g.                                                   Total amount of Ni.sup.2+  lost in                                                                16.07 g.                                                  process water and wash water                                                  Calculated amount of lost                                                                         43.44 g.; 18.2%                                           Ni(HNCN).sub.2.H.sub.2 O                                                      ______________________________________                                    

It is remarkable that 18% of the final product is not recoverable, beingdissolved in the process water, even at optimal Ni/H₂ NCN=1:2.2 molarratio. As above indicated, the low yield and related consequences aredue to the fact that the precipitation reaction involving solublehexamine nickelate or cobaltate species respectively is based on theemployment of ammonia as the pH control reagent.

The practical embodiment of the present invention is illustrated in 6examples as follows:

EXAMPLE 2

Pigment grade cobalt bis-hydrogen cyanamide with enhanced corrosionpreventive activity symbolized by the structural formula Co(HNCN)₂ H₂O_(x) (where x=0-1) was produced according to the following procedure:

A cobalt salt solution (A) was prepared by solubilizing 1 mole (263.06g.) technical grade CoSO₄ 6H₂ O (from Hydrite Chemical Co. of Wisconsin)in 700 ml. water by stirring it at ambient temperature. Water was addedto adjust the volume of the Co²⁺ solution to 800 ml.

Solution (B) containing 2.2 moles of hydrogen cyanamide was prepared byintroducing 185.0 g. of stabilized aqueous hydrogen cyanamide solutionof 50% by weight concentration (available from Cyanamide Canada Inc.under the trade name of Cyanamide-50) into 100 ml. water of normaltemperature and by adjusting the solution's final volume to 500 ml.

A sodium hydroxide solution (C) of approximately 10% concentrationcontaining 2 moles of NaOH was prepared by introducing 160 g. technicalgrade caustic solution of 50% by weight into 400 ml. cold water, coolingit subsequently to ambient temperature and by adjusting the volume to800 ml.

In order to prepare pigment grade cobalt bis-hydrogen cyanamide, thepreviously prepared (A), (B) and (C) solutions were introducedsimultaneously in approximately 30 minutes (applies to A and C), atidentical volumetric rate of approximately 25-30 ml/minute into 400 ml.intensively stirred water at normal temperature.

Substantial stoichiometrical excess of hydrogen cyanamide was assured inthe reaction system by carefully keeping the reactant solutions'delivery rate constant all during the precipitation as well as byproviding some initial excess of that reagent.

The slurry's pH was monitored accurately and kept rigorously betweenpH=8.0-9.0 and preferably at pH=8.5 by adjusting the NaOH (C) solution'sdelivery rate accordingly.

The precipitation was completed by stirring the slurry unheated for onehour at pH=8.5 and the process was completed by heating and stirring theslurry at 40° C.±5° C. for 30 minutes.

Subsequently, the product was washed to an essentially soluble salt-freecondition by repeated sedimentation of the slurry, decantation, freshwater introduction and reslurrying until a conductivity of 300micromhos/cm of the supernatant was reached; at that stage byCyanamide-50 addition, a concentration of 0.1% hydrogen cyanamide wasgenerated and the slurry was subsequently stirred for 15-30 minutes.

After solid-liquid separation, the pigment grade cobalt bis-hydrogencyanamide was obtained by drying it at 55°-65° C. for 12 hours to 4-8%H₂ O content and pulverizing it to fineness of 100%+230 mesh.

It is important to note that the mother liquor resulting from the firstdecantation of the slurry, as well as the wash water generated, wascolorless and contained only 0.5-1 mg/l dissolved CO²⁺.

Total amount of pigment grade product recovered was 154 g.

Analytical data obtained on dried product typical for pigment gradecobalt bis-hydrogen cyanamide produced according to the above describedprocedure are presented in Table 2.

                  TABLE 2                                                         ______________________________________                                        Analyzed or Tested                                                                            Value       Test Procedure                                    Parameter       Found       Applied                                           ______________________________________                                        Appearance      Light brown                                                                   powder                                                        Co as Co %      40.0        Complexometry                                     N as N %        37.2        Kjeldahl                                          Co bis-hydrogen 93.6        Calculated                                        cyanamide %                 value based on                                                                N %                                               H.sub.2 O %     5.2         Calculated                                        N/Co ratio      3.91                                                          Non-soluble impurities                                                                        1.2         Calculated                                        probable Co.sub.2 O.sub.3 %                                                   Conductivity    200 ± 25 ASTM D-281-31                                     (micromhos/cm)                                                                pH              9.0-9.2     ASTM D-1208-                                                                  84/6.1.1                                          Specific Gravity                                                                              2.23        ASTM D-113-84                                     Oil Absorption Lbs/100 Lbs                                                                    27-30       ASTM D-281-31                                     Yield           154 0 g.;                                                                     96.9% as                                                                      monohydrate                                                   Total amount of process water                                                                         Approx. 10 l.                                         (mother liquor & wash water)                                                  recovered                                                                     Co.sup.2+  concentration in process water                                                             <1 mg/l.                                              ______________________________________                                    

EXAMPLE 3

Pigment grade nickel bis-hydrogen cyanamide (symbolized by thestructural formula Ni(HNCN)₂ H₂ O_(x) where x=0-1) with enhancedcorrosion preventive activity suitable to be employed in "mirrorbacking" paint formulations was produced according to the procedurepursuant to Example 2, except that Solution (A) was prepared bydissolving 1 mole (262.7) of technical grade NiSO₄ 6H₂ O in a similarfashion, as presented in Example 2.

Total amount of pigment grade product recovered was 155.0 g.

It is observed that the mother liquor recovered from the firstdecantation of the slurry, as well as the wash water generated, wascolorless and contained 0.5-1 mg/l Ni²⁺.

Analytical data (obtained on dried product) typical for the pigmentgrade nickel bis-hydrogen cyanamide produced according to the procedureabove described are presented in Table 3.

                  TABLE 3                                                         ______________________________________                                        Analyzed or Tested                                                                            Value       Test Procedure                                    Parameter       Found       Applied                                           ______________________________________                                        Appearance      Light green                                                                   powder                                                        Ni as Ni %      38.4        Complexometry                                     N as N %        36.3        Kjeldahl                                          Ni bis-hydrogen 91.2        Calculated                                        cyanamide %                 value based on                                                                N %                                               H.sub.2 O %     8.3         Calculated                                        N/Ni ratio      3.96                                                          Non-soluble impurities                                                                        ˜0.5  Calculated                                        probable NiO %                                                                Conductivity    200 ± 25 ASTM D-281-31                                     (micromhos/cm)                                                                pH              9.0-9.3     ASTM D-1208-                                                                  84/6.1.1                                          Specific Gravity                                                                              2.22        ASTM D-113-84                                     Oil Absorption Lbs/100 Lbs                                                                    27-30       ASTM D-281-31                                     Yield           155.0 g.;                                                                     97.6% as                                                                      monohydrate                                                   Total amount of process water                                                                         9-10 l.                                               recovered                                                                     Ni.sup.2+  concentration in process                                                                   <1 mg Ni.sup.2+ /l.                                   water                                                                         ______________________________________                                    

EXAMPLE 4

Coprecipitated pigment grade Co-Ni bis-hydrogen cyanamide of 1:1 withenhanced corrosion preventive activity, suitable to be employed inmirror backing paint system formulations, was produced according to theprocedure pursuant to Example 2, except that Solution (A) was preparedby dissolving 0.5 mole of each CoSO₄ 6H₂ O (131.45 g.) and NiSO₄ 6H₂ O(131.35 g.) in a similar fashion, as presented in Example 2.

Total amount of pigment grade product recovered (before grinding) was152.0 g.

It is observed that the mother liquor recovered from the firstdecantation of the slurry was, as well as the wash water generated,colorless and contained 0.5-1 mg/l. Co²⁺ and Ni²⁺.

Typical analytical data (obtained on dried product) characterizing thecoprecipitated pigment grade product obtained according to theabove-presented procedure, are presented in Table 4.

                  TABLE 4                                                         ______________________________________                                        Analyzed or Tested                                                                            Value       Test Procedure                                    Parameter       Found       Applied                                           ______________________________________                                        Appearance      Light brown                                                                   powder                                                        Ni.sup.+  & Co %                                                                              38.6        Complexometry                                     N as N %        36.7        Kjeldahl                                          Ni bis-hydrogen 92.3        Calculated                                        cyanamide %                 value based on                                                                N %                                               H.sub.2 O %     7.6         Calculated                                        Ni/(Co + Ni) ratio                                                                            3.99                                                          Non-soluble impurities                                                                        <0.2        Calculated                                        (probable NiO + Co.sub.2 O.sub.3) %                                           Conductivity    200 ± 50 ASTM D-281-31                                     (micromhos/cm)                                                                pH              8.6-8.9     ASTM D-1208-                                                                  84/6.1.1                                          Specific Gravity                                                                              2.23        ASTM D-113-84                                     Oil Absorption, 27-30       ASTM D-281-31                                     Lbs/100 Lbs.                                                                  Yield           152.0 g.;                                                                     95.7% as                                                                      monohydrate                                                   Total amount of process 10-11 l.                                              water recovered                                                               Ni.sup.2+  + Co.sup.2+  concentration in process                                                      <1 mg/l.                                              water                                                                         ______________________________________                                    

EXAMPLE 5

Coprecipitated Ni²⁺ bis-hydrogen cyanamide mono-hydrite/zinc cyanamidepigment system with 1:10 molar ratio, characterized by enhancedcorrosion preventive activity on silver substrate and applicable inmirror backing formulations was produced according to the procedure, asfollows:

A combination (A) of a soluble nickel salt solution-zinc oxidesuspension was prepared by introducing 0.1 moles of NiSO₄ 6H₂ O (26.3g.) as 30% aqueous solution, into a previously prepared ZnO suspensionof 10-12%, containing 1.0 mole (81.4 g.) of well dispersed and hydratedhigh grade ZnO at normal temperature and by adjusting the total volumeto 600 ml.

The above-mentioned ZnO suspension was obtained by introducing, in smallincrements, 81.4. of finely ground ZnO (Azo 66 grade, with averageparticle size of 0.25 microns and 99.8% assay, manufactured by Asarco,Inc. was used) into 400 ml. intensively stirred hot water at 75°-80° C.and by cooling it to ambient temperature after one hour.

Solution (B) containing 1.32 moles of hydrogen cyanamide was prepared byintroducing 110.9 g. Cyanamide-50 into 250 ml. water and by completingthe solution's volume to 400 ml.

A sodium hydroxide solution (C) of approximately 16% concentration,containing 10.2 moles of NaOH was prepared by introducing 16.0 g.technical grade caustic solution of 50% by weight into 50 ml. coldwater, cooling it subsequently to normal temperature and by completingthe volume to 100 ml.

Coprecipitated pigment system of Ni²⁺ bis-hydrogen cyanamidemono-hydride/zinc cyanamide at 1:10 molar ratio was produced bysimultaneous introduction, at identical delivery rate, of solutions (A)and (B) into 200 ml. intensively stirred water, at normal temperature,respectively by adjusting the delivery rate of solution (C) accordinglyto keep the reaction system's pH=8-9.0, and preferably at pH=8.5, allduring the reaction.

By keeping the delivery rate of Solution (A) and (B) rigorouslyidentical, substantial stoichiometrical excess of H₂ NCN was kept in thereaction system all during precipitation.

The reaction was completed by stirring for two hours and adjusting theslurry's pH periodically to pH=8.5 by diluted H₂ SO₄ addition, and byheating it at 40° C.±3° C. for 30 minutes. Subsequently, the resultantcoprecipitated product was processed in a similar fashion as presentedat Example 2.

The total amount of pigment grade coprecipitated product recovered was120.5 g. The resultant mother liquor and wash water (total amount of6.01) was colorless and contained less than 0.5 mg. Ni²⁺ /l. Analyticaldata relevant to such coprecipitated pigment system of Ni(HNCN)₂ H₂O/ZnNCN with 1:10 molar ratio are presented in Table 5.

                  TABLE 5                                                         ______________________________________                                        Analyzed or Tested                                                            Parameter           Found Values                                              ______________________________________                                        Appearance          White-faint green                                                             powder                                                    Zn as Zn %          52.82                                                     Ni as Ni %          4.95                                                      N as N %            23.94                                                     ZnNCN + 0.1 Ni (HNCN).sub.2.                                                                      71.2-Calculated                                           H.sub.2 O %                                                                   Oxides (ZnO, NiO) % 21.6-Calculated                                           H.sub.2 O %         7.2-Calculated                                            Zn/Ni ratio         9.58                                                      N/(Zn + Ni) ratio   1.91                                                      Conductivity        350-600                                                   (micromhos/cm)                                                                pH                  8.0-8.5                                                   Specific Gravity    2.80                                                      ______________________________________                                    

EXAMPLE 6

Co²⁺ bis-hydrogen cyanamide mono-hydrite/zinc cyanamide coprecipitatedpigment system with 1:10 molar ratio applicable in mirror backingformulations, was produced in an identical manner as presented inExample 5, except that instead of NiSO₄ 6H₂ O, Solution (A) contained0.1 moles (23.6 g.) of technical grade CoSO₄ 6H₂ O.

The resultant process water was colorless and contained less than 0.5mg. Co²⁺ /l. A total amount of 120.0 g. of coprecipitated product wasrecovered. The relevant analytical data are presented in Table 6.

                  TABLE 6                                                         ______________________________________                                        Analyzed or Tested                                                            Parameter           Found Values                                              ______________________________________                                        Appearance          Light brown powder                                        Zn as Zn %          52.95                                                     Co as Co %          4.86                                                      N as N %            24.21                                                     ZnNCN + 0.1 Co      72.0-Calculated                                           (HNCN).sub.2.H.sub.2 O %                                                      H.sub.2 O %         6.4-Calculated                                            Oxides (ZnO, Co.sub.2 O.sub.3) %                                                                  21.6-Calculated                                           Zn/Co ratio         9.82                                                      N/(Zn + Co) ratio   1.93                                                      Conductivity        350-600                                                   (micromhos/cm)                                                                pH                  8.0-8.6                                                   Specific Gravity    2.78                                                      ______________________________________                                    

EXAMPLE 7

Co²⁺ and Ni²⁺ bis-hydrogen cyanamide mono-hydrite/zinc cyanamidecoprecipitated pigment system with 0.05:0.05:10 molar ratio, applicablein mirror backing formulations, was prepared in identical manner aspresented in Example 5, except that (A) contained 0.05 moles (131.1 g.)of each, NiSO₄ 6H₂ O and CoSO₄ 6H₂ O.

A total amount of 120.5 g. of coprecipitated product was recovered. Theresulted process water was colorless and contained less than 0.5 mg./l.of Ni²⁺ and Co²⁺ species. The relevant analytical data are presented inTable 7.

                  TABLE 7                                                         ______________________________________                                        Analyzed or Tested                                                            Parameter           Found Values                                              ______________________________________                                        Appearance          Light brown powder                                        Zn as Zn %          52.87                                                     Ni and Co %         4.91                                                      N as N %            24.20                                                     ZnNCN + 0.1 (CO + Ni)                                                                             72.0-Calculated                                           (HNCN).sub.2.H.sub.2 O %                                                      Zn/(Co + Ni) ratio  9.68                                                      N/(Zn + Co + Ni) ratio                                                                            1.93                                                      H.sub.2 O %         7.2                                                       Oxides (ZnO, Co.sub.2 O.sub.3,                                                                    20.8                                                      NiO) %                                                                        Conductivity        350-600                                                   (micromhos/cm)                                                                pH                  8.0-8.6                                                   Specific Gravity    2.79                                                      ______________________________________                                    

The corrosion retardant materials of this invention can be incorporatedinto a mirror backing composition containing an organic film formingresin which is dispersed or dissolved in either water or an organicsolvent. The organic film forming resin should be one which iscompatible with the thin metallic mirror film and does not promotedeterioration and discoloration of the film. Thus, the resin should notcontain functional groups which are reactive with the metal. Preferredorganic film forming resins include alkyd resins, acrylic resins,acrylic modified alkyd resins, polyurethane oils, vinyl halide polymersor copolymers, epoxy melamine or urea resins, non-oil based urethane,phenol-formaldehyde resins curable by air drying or baking, or any otherresin which is compatible to the metallic layers formed on mirrorbackings. Examples of solvents for such compositions include xylene,MIBK or toluene. The composition can also contain other components suchas catalysts, flow control agents, dryers, anti-settling agents, and thelike and mixtures thereof.

What is claimed is:
 1. A method of preparing a (M)bis-hydrogen cyanamidecomposition with enhanced corrosion preventive activity, wherein M is adivalent metal selected from the group consisting of nickel, cobalt ormixtures thereof comprisingprecipitating metal hydroxides selected fromthe group consisting of Ni(OH)₂, Co(OH)₂ or mixtures thereof, in situ ina liquid reaction medium, containing an excess of H₂ NCN and immediatelyreacting the resultannt precipitate with said H₂ NCN.
 2. A methodaccording to claim 1 wherein said reaction is carried out for 15 to 30minutes, and the alkalinity and temperature of the reaction medium ismaintained at about pH=7.5 to 9.2, and 15°-25° C., respectively.
 3. Amethod according to claim 2 wherein the pH is maintained at 8.5.
 4. Amethod according to claim 1 wherein cobalt and nickel bis-hydrogencyanamides are co-precipitated.
 5. A method according to claim 1 whereinsaid bis-hydrogen cyanamide composition is co-precipitated with zinccyanamide by adding a zinc oxide suspension to said liquid reactionmedium.
 6. A method according to claim 5 wherein nickel bis-hydrogencyanamide, cobalt bis-hydrogen cyanamide and zinc cyanamide areco-precipitated from said reaction medium.
 7. A method according toclaim 1 wherein said precipitation step is conducted by simultaneousintroduction into the reaction medium of a soluble nickel or cobalt saltand an alkali metal hydroxide.
 8. A method according to claim 7 whereina stoichiometric excess of hydrogen cyanamide is maintained in thereaction medium throughout the precipitation step.
 9. A method accordingto claim 8 wherein the molar ratio between the metal hydroxides and H₂NCN is 1/2.2.
 10. A method according to claim 1 comprising the furthersteps of filtering the reaction product, washing the resulting productwith water to a substantially salt-free condition and subsequentlydrying said product.